Deflection circuit with vdr feedback means



Feb. 1, 1966 M. MEYER DEFLECTION CIRCUIT WITH VDR FEEDBACK MEANS FiledJune 28, 1962 INVENTOR MANFRED MEYER AGENT United States Patent 9Claims. (a. 315-27 This invention relates to circuit arrangements forproducing a sawtooth current in a coil by means of a discharge tubehaving a control grid to which is applied a voltage which periodicallyrenders the tube conductive, the output circuit of the tube comprising atransformer to which the coil is coupled and a linearizing diodecircuit. The pulses occurring at the transformer during the flyback ofthe sawtooth current are applied through a coupling capacitor to aresistor having a non-linear symmetrical voltage vs. currentcharacteristic curve, across which a direct voltage occurs which is fedto the control circuit, for example,-the grid of the tube.

When applying the available pulse voltage occurring at the transformerto said non-linear resistor, one cannot be certain that without the useof additional means the correct value of the direct control-voltage canbe obtained which is necessary for the desired adjustment of theworking-point of the tube. Consequently, a voltage-divider circuit isgenerally to be included, for example, a capacitor circuit dividing theamplitude of said pulse voltage. It may be required for this voltagedivider to be made adjustable, in particular with a view to compensationof the tolerances of the associated structural elements, especially whenusing a voltage dependent resistor as the non-linear resistor, sinceVDRs are subject to wide tolerances in their properties. In Britishpatent specification 890,805, in which a positive bias voltage isapplied to the non-linear resistor, a potentiometer circuit is connectedto a stabilized voltage source, more particularly the booster voltage,and a connection exists from a tapping point on said potentiometercircuit through an ohmic resistor to the non-linear resistor. By varyingthe bias voltage, variations in the non-linear resistor within itstolerances could also be compensated, but only to a certain proportion,since only a limited variation of the potentiometer circuit was possiblein view of the loading effect on the direct-voltage source and thereliability in operation of the direct-voltage potentiometer. The shiftof the working point of the non-linear resistor due to the variable biasvoltage otherwise also resulted in an undesirable variation of thecontrol characteristic curve because with unchanged amplitude of thepulsed voltage applied, a variation in the bias results in an unwantedvariation of the grid-bias derived for the tube.

In order to obviate these disadvantages, it has previously beensuggested to leave the connection between the non-linear resistor andthe pulse voltage source unchanged and to obtain the required adjustmentby connecting a voltage divider, preferably adjustable, in parallel withthe non-linear resistor. The tapping of the voltage divider is thenconnected to the control grid of the tube. 'This tapping may also beconnected to another part of the circuit controlling the tube, wherebythe required increase of energy supply in the output circuit of the tubemay be controlled. The said circuit previously suggested may also beused if, besides the pulsed voltage, a direct voltage for the adjustmentof the workingpoint is applied to the non-linear resistor, which resultsin an improvement in the control sensitivity.

Suitable non-linear resistors preferably have non-linear 3,Z33,ld3Patented Feb. 1, i966 symmetrical characteristic curves, which can bedescribed by the equation U=K I", in which U is the voltage applied to,and I the current through the non-linear resistor, whereas K is aproportionally constant and B an exponent. It should be noted that thevoltage U and the current I always have the same polarity and that onlythe absolute values of U and I should be inserted in the above givenequation. The exponent [3 has values of, for example, 0.15 to 0.25. Suchresistors may consist of silicon-carbide grains which are mixed with abonding agent to form a ceramic-like mass and then sintered. The voltagedependency is based on the variable contact resistance between theindividual carbide crystals. Such resistors are commercially availableunder the name of VDR- resistors. With a tolerance of il()% in thevoltage applied to said resistor there occur deviations in the controlvoltage obtained due to the action of the exponent s in a ratio of 1:7in the tolerance range. Consequently, the voltage divider which,according to the previous suggestion, was connected in parallel with thenon-linear resistor, must be variable substantially throughout its ohmicvalue and, more particularly, a potentiometer so arranged is exposed tothe full voltage, i.e. the alternating pulsed voltage, and must be ratedto withstand this voltage. This involves considerable difiiculties inpractice.

In a circuit arrangement of the kind mentioned in the preamble, thesedisadvantages are avoided and with the use of a smaller number ofstructural elements which are subjected only to a slight load. A widecompensation range and a reliable and more accurate control are obtained if, according to the invention, the internal resistance of thepulse voltage source is varied.

This is possible by suitably influencing the circuit or by choosing thecoupling capacitor through which the pulsed voltage is transmitted ofthe variable type.

Preferably, in accordance with the invention, a variable resistor isconnected in series with the coupling capacitor. When voltage dividersare employed, for example, at the direct-current side or at the pulsesupply side, resistors of comparatively low values must be used whichconstitute an undesirable load on the circuit arrangement. However, whenusing a series-resistor, the circuit may be proportioned so that theload is a minimum at one limit of the tolerance range at which acomparatively low direct voltage only is produced, whereas at the otherlimit of the tolerance range, the series-resistor is made active andhence the load impedance is increased. Since the non-linear resistorabsorbs a particularly high current at high voltages and just the peakvalue is a measure of the control voltage obtained, the use of aseries-resistor is particularly effective and this resistor may be of acomparatively lowvalue.

In order that the invention may be readily carried into effect, it willnow be described more fully, by way of example, with reference to theaccompanying diagrammatic drawing.

The figure shows the output stage of a circuit arrangement for thehorizontal deflection of the electron beam of the picture tube in atelevision receiver. A control voltage 2 is applied through a couplingcapacitor 3 to the control grid 5 of an amplifier tube 1, for example, apentode. The anode current of tube 1 flows through the primary winding 7of a transformer 8 and produces, together with a series-booster diode 6having a well known linearising effect, a sawtooth current through adeflection coil 10 which is connected to a tap on the winding 7. One endof the winding 7 is connected to ground through a capacitor 9 associatedwith the series-booster diode circuit. The cathode of diode 6 isconnected to a further tap on the winding 7 and its anode is connectedto the positive terminal of a supply voltage source of, for

example, 220 volts. Moreover, a diode 12 is connected to thehigh-voltage secondary winding 11 of transformer 8. The diode 12supplies a direct voltage for the output anode of a display tube (notshown) to a capacitor 13.

A voltage which exhibits positive going pulses having an amplitude of,for example, 1,250 volts during the flyback of the sawtooth current, isapplied from a suit able tap on the primary 7 through a capacitor 15 anda variable resistor 14, included in accordance with the invention, to anon-linear or VDR-resistor 16, the other end of which is connected toground. This resistor has the above-mentioned voltage-currentcharacteristic curve U=K I", wherein K may have a value of 2,600 and ,8a value of 0.175. The coupling capacitor 15 may have a value of, forexample, 270 pi. and the resistor 14 may have a linear characteristiccurve and a value of 250,000 ohms. The VDR-resistor 1.6 is connectedthrough a resistor 18 of, for example, 13.6 megohms to the capacitor 9across which a direct current boost voltage of approximately 800 voltsoccurs.

A pulsed voltage is set up at the V DR-resistor l6 having a peak valuewhich amounts to, for example, from 1,100 to 1,400 volts. Theworking-point of resistor 16 is shifted onto the positive branch by thedirect voltage from resistor 18. A mean direct voltage of, for example,60 volts with respect to ground, is finally set up across resistor 16,which mean voltage results from the combined action of the rectificationof the pulses applied to resistor 16 and a shift due to the positivebias voltage applied to it. If resistor 14 is not included in thecircuit, said direct voltage may assume values between 60 and 240 voltsas a result of the wide tolerances in the properties of VDR 16. Thesegreat deviations may be satisfactorily compensated by the specifiedvalue of resistor 14, which is preferably active during the peaks of thepulses and exerts a current-limiting effect.

In order to balance the circuit, an ohmic resistor 1'7 of the order of afew hundred thousands of ohms may be connected in parallel with thenon-linear resistor 16.

The pulsed voltage may alternatively be matched to the requirements ofthe control circuit by connecting a capacitor 19 of about 20 to 50 pf.relative to earth between the junction of coupling capacitor 15 andvariable resistor 14. However, said circuit element may generally beomitted.

With the series-resistor 18 connected as illustrated, it has applied toit approximately half the pulsed voltage. In the arrangement shown,however, a corresponding direct-current potential is also applied to thetap to which the coupling capacitor 15 is connected. Consequently, analternating-voltage load on resistor 18 may be avoided if the free endof resistor 18 is connected to the same tap as said capacitor. It willbe evident that the other end of resistor 18 may also be connected tothat end of resistor 14 which is connected to coupling capacitor 15. Inthis case, the series-resistor 1S and the capacitor 15 are connected inparallel so that the resistor 18 is for all practical purposes uncoupledfor alternating voltages, and thus loaded only minimally.

In principle, the direct voltage supplied through resistor 18 could bederived from an arbitrary voltage source of suitable value. If, forexample, the supply source is used, it appears that fluctuations of thesupply source are introduced into the control circuit and may produceinterference therein. Such deviations need not be feared, if the voltageis derived from the series-booster diode circuit, since the stabilizingcircuit also keeps the voltage across capacitor 9 substantiallyconstant. The fluctuations of the capacitor voltage may thus give riseonly to a certain decrease in control sensitivity, but externalinterferences can not influence the control action.

The supply source can then exercise a deterimental eitect only as far asit constitutes a fixed part of the voltage across capacitor 9. It istherefore advantageous to derive the direct voltage from a point of thecircuit arrangement which belongs substantially to the linearizing diodecircuit only, and not from the supply voltage.

By the use of a direct bias voltage (across resistor 18), a substantialincrease in control sensitivity, that is to say of the variation indirect voltage (across resistor 16) is obtained for a given percentagevariation in the amplitude of the pulses. This control sensitivityvaries only to an inappreciable extent if resistor 14- is adjusted forcompensation of the tolerances to the same direct basic voltage (biasvoltage for the grid of tube 1). Consequently the operation of thecircuit arrangement is independent of the tolerance.

It will be evident that the idea of the invention is also applicable toa circuit arrangement in which amplifier elements of a difierent kind,for example, transistors, are used.

What is claimed is:

1. A circuit for producing in a coil a sawtooth current having alinearly varying portion and a flyback portion, comprising an amplifierdevice having a control electrode and .an output electrode, atransformer having winding means, means connecting said transformerwinding to said output electrode and to a source of voltage, means forap plying an electric signal to said control electrode for periodicallyrendering said amplifier device conductive whereby said sawtooth currentis established in said winding means, means coupling said coil to saidwinding means, a control circuit comprising an output terminal coupledto said amplifier control electrode and a resistor having a symmetricalnon-linear voltage-current characteristic curve for developing a directcurrent voltage at said output terminal, a capacitor connected betweensaid winding means and said control circuit for coupling fiyback pulsesfrom said winding means to said control circuit, said non-linearresistor producing a direct current voltage which varies with theamplitude of said fiyback pulses, and means for adjusting the amplitudeof the flyback pulse applied to said non-linear resistor comprising avariable resistor connected in series with said capacitor.

2. Apparatus as described in claim 1 further comprising means forsupplying a direct current voltage to said non-linear resistor of thesame polarity as said flyback pulses.

3. Apparatus as described in claim 2 wherein the amplitude of saiddirect current voltage is of the same order of magnitude as theamplitude of said flyback voltage pulses.

Apparatus as described in claim 2 wherein said nonlinear resistor iscomposed of a material which exhibits a variation in resistance with avariation in the voltage applied thereto, said apparatus furthercomprising an ohmic resistance element connected in parallel with saidnon-linear voltage dependent resist-or.

5. A circuit for producing a sawtooth current in a coil, comprising anamplifier device having a control electrode and an output electrode, acapacitor, a transformer winding connected in series between said outputelectrode and said capacitor, means for applying an electric signal tosaid control electrode for periodically rendering said amplifier deviceconductive whereby a linearly varying current and a ilyback voltagepulse are developed in said winding, said capacitor being charged bysaid fiyback pulses to produce a direct current boost voltage therein,means coupling said coil to said transformer winding, 21- controlcircuit comprising a device having a symmetrical. non-linearvoltage-current characteristic curve, a coupling capacitor connectedbetween said transformer winding; and said non-linear device wherebysaid flybck pulses are: coupled to said non-linear device, saidnon-linear device: producing a direct current voltage which varies withthe amplitude of said liyb-ack pulses, a variable resistor connected inseries with said coupling capacitor and said nonlinear device, directcurrent coupling means for applying said capacitor direct current boostvoltage to the series combination of said variable resistor and saidnon-linear resistor, and means for supplying said direct current voltageproduced by said non-linear device to said control electrode so as tooppose a variation in the amplitude of said sawtooth current.

'6. Apparatus as described in claim 5 further comprising a damping diodecoupled to said transformer win-ding and to a source of direct currentvoltage for improving the linearity of said sawtooth current flowing insaid transformer winding, said direct current coupling means beingconnected to supply a direct current voltage which is derived only fromthe damping diode circuit.

7. A circuit for producing a sawtooth current in a coil, comprising anamplifier device having a control electrode and an output electrode, acapacitor, a transformer winding connected in series between said outputelectrode and said capacitor, means for applying an electric signal tosaid control electrode for periodically rendering said amplifier deviceconductive whereby a sawtooth current and a flyback voltage pulse aredeveloped in said Winding, said flyback pulse producing a direct currentvoltage on said capacitor, means coupling said coil to said transformerwinding, a control circuit comprising a voltage dependent resistorhaving a symmetrical non-linear voltagecurrent characteristic and anadjustable resistor connected in series, a coupling capacitor forcoupling said flyback pulse to said control circuit, means connectingsaid coupling capacitor in series with said voltage dependent resistorand said adjustable resistor and to a tap on said transformer winding atwhich said flyback voltage pulse and said direct current voltage appear,said voltage-dependent resistor producing a direct current negativefeedback voltage which varies with the amplitude of the flyback pulsesapplied thereto, and means for supplying said direct current voltageproduced by said voltage dependent resistor to said control electrodethereby to stabilize the amplitude of said sawtooth current.

8. Apparatus as described in claim 7 further comprising a resistorconnected in parallel with said coupling capacitor, the resistance ofsaid latter resistor being substantially greater than the maximumresistance value of said adjustable resistor.

*9. A circuit for producing a sawtooth current in a coil, comprising anamplifier device having a control electrode and an output electrode, acapacitor, a transformer having a primary winding and .a secondarywinding, means connecting said primary winding in series between saidoutput electrode and said capacitor, means for applying an electricsignal to said control electrode for periodically rendering saidamplifier device conductive whereby a sawtooth current and .a flybackvoltage pulse are developed in said primary winding, means coupling saidcoil to a tap on said primary winding, a voltage dependent resistorhaving a symmetrical non-linear voltage-current characteristic curve, avariable resistor, a coupling capacitor, means for connecting saidvoltage dependent resistor, said variable resistor and said couplingcapacitor in series between a point of reference potential and a tap onsaid primary winding in the order recited, a resistor connected betweensaid first named capacitor and the common junction of said voltagedependent resistor and said variable resistor to supply a .positivedirect current bias voltage to said voltage dependent resistor, saidvoltage dependent resistor producing a negative direct current voltageat said common junction Which'varies with the amplitude of said flybackpulses, a resistor connected between said common junction and saidcontrol electrode, an rectifier means connected to said secondarywinding for producing a direct voltage.

1. A CIRCUIT FOR PRODUCING IN A COIL A SAWTOOTH CURRENT HAVING ALINEARLY VARYING PORTION AND A FLYBACK PORTION, COMPRISING AN AMPLIFIERDEVICE HAVING A CONTROL ELECTRODE AND AN OUTPUT ELECTRODE, A TRANSFORMERHAVING WINDING MEANS, MEANS CONNECTING SAID TRANSFORMER WINDING TO SAIDOUTPUT ELECTRODE AND TO A SOURCE OF VOLTAGE, MEANS FOR APPLYING ANELECTRIC SIGNAL TO SAID CONTROL ELECTRODE FOR PERIODICALLY RENDERINGSAID AMPLIFIER DEVICE CONDUCTIVE WHEREBY SAID SAWTOOTH CURRENT ISESTABLISHED IN SAID WINDING MEANS, MEANS COUPLING SAID COIL TO SAIDWINDING MEANS, A CONTROL CIRCUIT COMPRISING AN OUTPUT TERMINAL COUPLEDTO SAID AMPLIFIER CONTROL ELECTRODE AND A RESISTOR HAVING A SYMMETRICALNON-LINEAR VOLTAGE-CURRENT CHARACTERISTIC CURVE FOR DEVELOPING A DIRECTCURRENT VOLTAGE AT SAID OUTPUT TERMINAL, A CAPACITOR CONNECTED BETWEENSAID WINDING MEANS AND SAID CONTROL CIRCUIT FOR COUPLING FLYBACK PULSESFROM SAID WINDING MEANS TO SAID CONTROL CIRCUIT, SAID NON-LINEARRESISTOR PRODUCING A DIRECT CURRENT VOLTAGE WHICH VARIES WITH THEAMPLITUDE OF SAID FLYBACK PULSES, AND MEANS FOR ADJUSTING THE AMPLITUDEOF THE FLYBACK PULSE APPLIED TO SAID NON-LINEAR RESISTOR COMPRISING AVARIABLE RESISTOR CONNECTED IN SERIES WITH SAID CAPACITOR.